1. Field of the Invention
The invention concerns a polarization-maintaining optical fiber that provides an improved single-mode, polarization-maintaining fiber coupler or coupler/splitter (herein simply called "coupler"). The invention also concerns a method of making the coupler and a mount for the coupler The preservation of polarization is especially important in fiber gyroscopes, interferometric sensors, and coherent communications.
2. Description of the Related Art
A single-mode optical fiber typically has a glass inner core of high index of refraction and a glass jacket of low index of refraction. Usually, the diameter of the core is from 3 to 10 micrometers, and the diameter of the jacket is 80 micrometers for sensor fibers and 125 micrometers for telecommunications. According to Shibata et al.: "Fabrication of Polarization-Maintaining and Absorption-Reducing Fibers," Journal of Lightwave Technology, Vol. LT-1, No. 1, pp. 38-43 (1983):
"The general approach to maintaining linear polarization in single-mode fibers is to increase fiber birefringence so as to reduce the power interchange between polarization modes. Several kinds of highly birefringent single-mode fibers have been demonstrated: fibers with a noncircular core, which cause birefringence due to noncircular geometry [citing Ramaswamy et al.: `Polarization Characteristics of Noncircular Core Single-mode Fibers,` Applied Optics, Vol. 17, No. 18, pp 3014-3017 (1978)]; fibers with an elliptical cladding, which cause anisotropic strains in the core [citing Ramaswamy et al.: `Birefringence in Elliptically Clad Borosilicate Single-mode Fibers,` Applied Optics, Vol. 18, No. 24, pp 4080-4084 (1979) and Katsuyama et al.: `Low-loss Single Polarization Fibers,` Electron. Lett., Vol. 17, No. 13, pp 473-474 (1981)]; and fibers with refractive-index pits on both sides of the core [citing Hosaka et al.: `Single-mode Fiber with Asymmetrical Refractive Index Pits on Both Sides of Core,` Electron. Lett., Vol. 17, No. 5, pp 191-193 (1981)]." PA1 "were fabricated with jacketing techniques. A single-mode fiber preform made by the VAD method [citing Tomaru et al.: `Fabrication of Single-mode Fibers by V.A.D.,` Electronics Lett., Vol. 16, No. 13, pp 511-512 (1980)]was elongated to several millimeters in diameter. Then, it was put into the center of a thick-wall jacketing silica tube with about 15-mm inner diameter. Stress-applying parts were prepared by a depositing SiO.sub.2 --B.sub.2 O.sub.3 --GeO.sub.2 glass layer in a silica tube via the MCVD method. The rods prepared by the MCVD method were also elongated to several millimeters and arranged on both sides of the core rod in the jacketing tube. The remaining inner spaces in the jacketing tube were filled with several commercially available silica rods, for example, four rods with several millimeters of diameter. The final preforms were drawn into fibers by a carbon-resistance furnace." PA1 "In order to reduce the transmissions loss, the stress-applying parts must be sufficiently separated from the core." PA1 has a thermal coefficient of expansion that PA1 substantially contacts the core, and PA1 has an area less than 10 percent that of the fiber. PA1 (1) placing two short lengths of the optical fiber side-by-side, PA1 (2) aligning the polarization axes of the two lengths, PA1 (3) fusing the lengths together, and PA1 (4) while launching light into one of the lengths, drawing the fused lengths until light being emitted from the lengths reaches a predetermined relationship, usually when equal light is emitted from the fused lengths.
The Shibata publication concerns a birefringent single-mode fiber, the silica jacket of which includes two fan-shaped, diametrically-opposed regions that have been doped to have a different thermal expansion than does the rest of the jacket, resulting in anisotropic stress-induced birefringence in the core. At page 40, the Shibata publication says these fibers
A birefringent fiber having similarly-shaped stress-applying regions as well as an elliptical core is shown in U.S. Pat. No. 4,480,897 (Okamoto et al.). FIGS. 6 and 8 of U.S. Pat. No. 4,561,871 (Berkey) also illustrate the manufacture of birefringent fibers having diametrically-opposed stress-applying regions separated from the core.
The Shibata publication says:
Some earlier patents did not recognize the need for such separation; see, U.S. Pat. No. 4,179,189 (Kaminow et al.) and U.S. Pat. No. 4,274,854 (Pleibel). Others such as U.S Pat. No. 4,478,489 (Blankenship) explain that if such separation is too small, there will be light transmission loss due to scattering. The Blankenship patent says that the separation should be as small as possible and indicates that when the separating material and the stress-applying regions have the same refractive index, the minimum radius to the stress-applying regions can be as small as 1.5 times the radius of the core.
Sasaki: "Long-Length Low-Loss Polarization-Maintaining Fibers," Journal of Lightwave Technology, Vol. LT-5, No. 9, pp 1139-1146 (1987), also concerns birefringent fiber having two diametrically-opposed stress-applying regions which it says should be separated from the core and that the absorption loss is satisfactorily low when the minimum radius to the stress-applying regions (r) and the radius of the core (a) have a ratio of more than 3.4. FIG. 7 of the Sasaki publication illustrates the manufacture of the fiber by forming two pits in the jacket and inserting doped rods into the pits that provide the stress-applying regions.
Katsuyama et al.: "Low-loss Single Polarization Fibers," Applied Optics, Vol. 22, No. 11, pp 1741-1747, (1983), concerns a polarization-maintaining optical fiber that is similar to that of the above-cited Ramaswamy publications in that the stress-applying region is elliptical and is the intermediate of three concentric silica regions that make up the jacket. The intermediate region is made stress inducing by being doped with B.sub.2 O.sub.3. The Katsuyama publication calls this intermediate region the "elliptical-jacket." The intermediate stress-applying region also is doped with GeO.sub.2 in order to make its refractive index the same as in the inner and outer of the three concentric regions. Consistent with the above quotation from the Shibata publication, the Katsuyama publication explains that ratio of the "core radius" to the "clad radius" (i.e., minimum radius to the "elliptical jacket") must be less than 0.5 to minimize absorption loss.
Three polarization-maintaining optical fibers are illustrated in U.S. Pat. No. 4,515,436 (Howard et al.). The fiber of FIG. 3 has a "highly elliptical inner cladding layer 32" that may comprise boron-doped SiO.sub.2 which is surrounded by an elliptical "outer cladding layer 34" that may comprise fluorine-doped SiO.sub.2 within a jacket that apparently is undoped silica. The large ellipticity of the "inner cladding layer thereby induces a large stress on the fiber, sufficient to significantly separate the two polarizations of the fundamental mode" (Column 4, lines 47-50).
Even though the stress-applying regions of some of the above-discussed prior polarization-maintaining fibers, such as that of FIG. 3 of the Howard patent, are not separated from the core, it is believed that all polarization-maintaining fibers currently on the market have such a separation and that (as taught in the Katsuyama publication) the ratio of the radius of the core to the minimum radius to the stress-applying regions is always less than 0.5 (typically about 0.2). Otherwise, a large proportion of light transmitted by the core would be absorbed by the doped, stress-producing region.
Important applications for polarization-maintaining fibers require a coupler that maintains the polarization. Numerous couplers have been reported. Villarruel et al.: "Fused Single-mode-fibre Access Couplers," Electronics Lett., Vol. 17, pp 243-244 (1980), employs cladding removal by radially etching to within a few micrometers of the core diameter, but by removing the stress-inducing regions, this apparently destroys the polarization-maintaining property. Kawasaki et al., "Biconical-taper $ingle-mode Fiber Coupler," Opt. Lett., Vol. 6, pp 327-328 (1981), employs fusion and tapering without etching to produce a biconically-tapered-fused coupler. In Villarruel et al.: "Polarization Preserving Single-mode-fibre Coupler," Electronics Lett., Vol. 19, pp 17-18 (1983), etching, fusion and tapering are combined, with no attempt being made to align the polarization axes of the coupler fibers. Nevertheless, the polarization was said to be satisfactorily maintained, and this was attributed to the large inherent birefringence of the fibers.
Dyott et al.: "Polarization-holding Directional Coupler Made from Elliptically Cored Fibre Having a D Section," Electronics Lett., Vol. 19, pp 601 and 602 (1983), employs a fiber that has an elliptical core and a cladding that has a D-shaped cross section, with the core close to the flat side of the D. In order to make a coupler, a small amount of material is etched from the surface of both fibers over a short length, and the fibers are positioned in the bore of a glass tube with the etched region adjacent and the flats of the Ds facing each other. The glass tube is heated and pulled to draw down a central section at which the cores come very close together.
In the coupler of Kawachi et al.: "Fabrication of Single-polarization Single-mode-fibre Couplers," Electronics Let., Vol. 18, No. 22 (1982), polarization is maintained by fusing two side-by-side optical fibers that are similar to those o the above-cited Ramaswamy, Birch, and Shibata publications, the Okamoto and Berkey patents, and FIG. 1 of the Blankenship patent. That is, the fibers that have diametrically-opposed stress-applying regions separated from the core. Polarization is said to be maintained in any of three possible alignments, namely, when the principal axes of the stress-applying regions of the two fibers are parallel, collinear, or perpendicular.
Yokohama et al.: "Polarization-Maintaining Fibre Couplers with Low Excess Loss," Electronics Lett., Vol. 22, No. 18, pp 929 and 930 (1986), concerns polarization-maintaining couplers made by fusing two aligned fibers that also have diametrically-opposed stress-applying regions spaced from the core. As preferred in the Blankenship patent, the stress-applying regions and the surrounding jacket or cladding have the same index of refraction. The coupler is made by aligning the polarization principal axis of two side-by-side fibers, fusing them together, and elongating a portion of the fused region "until the prescribed coupler ratio was obtained." The Yokohama publication reports an "excess loss of less than 0.1 dB was easily achieved" and that the best value was 0.03 dB. It also reports polarization cross talk (also called "polarization extinction coefficient") of less than -30 dB. Both of these values are about an order of magnitude better than in any coupler that we have seen on the market.
Pleibel et al.: "Polarization-Preserving Coupler with Self-aligning Birefringent Fibres," Electronics Lett., Vol. 19, pp 825 and 826 (1983), forms a coupler from fibers which are birefringent by having a highly-doped elliptical region in the cladding. "The fibre, together with its acrylate coating, is bent and bonded into a 25 cm radius in a silica block. One side of the fibre and its coating are polished away while actively monitoring the light transmitted through the fibre. When the transmitted light begins to drop, the polishing is stopped, and the two halves are placed together to check the coupling. The process is repeated until the desired level of coupling is obtained." U.S. Pat. No. 4,564,262 (Shaw) concerns a similar process, but says nothing about monitoring light transmission during the process.
In U.S. Pat. No. 4,632,513 (Stowe et al.), parallel juxtaposition segments of a pair of single-mode optical fibers are fused together to form a coupler which is polarization insensitive.